Steel wheels, aluminum wheels, and the like are used as motor vehicle wheels to which tubes, tires, etc. are to be fitted. In particular, aluminum wheels are widely used because they are excellent in terms of lightness of weight, anticorrosiveness, better design, etc.
Aluminum wheels are usually coated with a primer coating composition, and then with transparent, colored, or lustrous thermosetting topcoat composition(s) one or more times for protection and appearance.
Therefore, primer coating compositions for aluminum wheels need to have excellent adhesion to both the wheel substrate and any topcoat such as a colored topcoat. After a powder primer coating composition is applied, a liquid primer coating composition may additionally be applied to improve the smoothness of the wheel substrate. In this case, the liquid primer coating composition needs to have excellent adhesion to the powder primer coat.
A topcoat formed on the primer coat of an aluminum wheel is required to have excellent appearance in terms of smoothness and luster, and excellent mar resistance so as to prevent scratches caused by skipping stones, etc., when the vehicle is moving. Moreover, if dust or the like adheres to the topcoat in the wheel coating line, it is necessary to repair by grinding down that portion and then reapplying the same coating composition. In this case, excellent adhesion of the reapplied coat, i.e., recoat adhesion is necessary. Furthermore, it is also necessary that coating defects such as sagging and popping be unlikely to occur even if the coat thickness is increased to enhance the aluminum wheel topcoat appearance.
Japanese Unexamined Patent Publication No. 2001-62391 discloses a coating method of forming a primer layer, a lustrous clear coating layer, and a top clear coating layer on an aluminum wheel, wherein the top clear coating layer is formed using a thermosetting acrylic resin coating composition. However, when a known thermosetting acrylic coating composition as used in this method is used, it is difficult to form a top clear coat with excellent appearance, mar resistance and recoat adhesion.